The present invention is generally related to the removal of undesirable components from polymerized monovinyl aromatic compounds and more particularly is directed to the removal of volatile materials at the end of the polymerization process for monovinyl aromatic compounds such as polystyrene, while incorporating options to improve and optimize critical polymer properties.
It is well known in the polymer industry that volatiles such as unreacted monomer, dimers, and trimers may be removed from newly polymerized monovinyl aromatic compounds by means such as vacuum distillation and flash devolatilization. The vacuum process subjects the polymer to sub-atmospheric conditions to draw out the volatile components and the flash process may involve vacuum as well as heat to further drive out the volatiles. In addition to vacuum and heat devolatilization techniques, another method known in the art is that of "stripping" of volatiles by the use of stripping agents such as steam.
Toekes U.S. Pat. No. 3,311,676 teaches the devolatilization of a rubber modified polystyrene by the use of a preheater, a heat exchanger, and a vacuum vessel which Toekes classifies as a phase separator. The preheater heats the volatile-containing polystyrene material into the range of 200.degree. to 240.degree. C. and the heat exchanger maintains the temperature of the material while subjecting it to a reduced pressure in the range of 15 to 50 mm Hg. The vacuum on the heat exchanger is a result of its connection to the phase separator which operates at a pressure of 5 to 100 mm Hg. Toekes describes the process wherein a foam is generated in the heat exchanger and flows into the phase separator thereby accelerating the removal of volatiles by a many-fold increase in surface area of the material. The Toekes apparatus and procedure claims a reduction in volatiles to approximately 0.5 to 1.5 percent by weight which corresponds to a volatile level of 5000 to 15,000 PPM. According to Toekes, the monomer and EB (ethylbenzene) level was reduced to "below 0.1%" which corresponds to 1000 PPM.
Newman U.S. Pat. No. 3,668,161 et al. discloses a method for separating volatiles from a polymer by passing the polymer through a first flash devolatilization zone maintained at a vacuum and heating the partially devolatilized composition, adding a foaming agent and then passing through a second devolatilization zone at reduced pressure to vaporize and remove remaining volatile constituents. Polystyrene materials devolatilized by this process according to examples contained in the patent ended up with residual styrene in the amount of 2000 PPM.
Mertzinger U.S. Pat. No. 3,865,672 dated Feb. 11, 1975 discloses a process to remove volatiles from a polymer solution by using a single stage vacuum evaporation system in which the temperature of the polymer solution increases in the direction of the flow. Metzinger is concerned primarily with polymers containing acrylonitrile or methacrylonitrile. The process is achieved in Metzinger by the use of a counter-flow heat exchanger combined with a vacuum devolatilization zone. The counter-flow heat exchanger thus allows the heat to increase in the polymer as it proceeds through the devolatilization zone. More particularly, Metzinger discloses a vertical tube bundle heat exchanger operating in a downflow configuration wherein the polymer solution is fed at the top, devolatilized during the downflow, and then removed at the bottom. Likewise, the temperature gradient maintained in the heat exchanger increases from top to bottom such that the devolatilized polymer is subjected to increasing temperatures as it flows through the devolatilizing system. According to Metzinger, the volatile content was reduced to 3000 PPM.
Hagberg U.S. Pat. No. 3,928,300 dated Dec. 23, 1975 discloses a process for devolatilizing polystyrene by subjecting it to a downflow "falling strand" devolatilizer. A falling strand devolatilizer consists of a shell-and-tube heat exchanger in a vertical configuration stationed in the upper portion of a "flash tank". The flash tank has a vapor pump communicating therewith to remove volatiles which are flashed out of the polymer. The vertical shell-and-tube heat exchanger comprises a series of parallel tubes held rigidly within the heat exchanger shell in a vertical orientation. Heated polymer flows through the tubes in response to gravity and the pressure differential established by the vapor pump which creates a vacuum in the flash tank. The heated polymer exits the tubes of the heat exchanger in strands or strings which then release the volatiles due to the combination of heat from the heat exchanger and the low pressure in the flash tank. The strands drop to the lower end of the conical flash tank and then are regulated by a plug valve and flowed downward into a hold tank which may also be used as a second flash tank. The Hagberg patent claims a method of controlling the volatiles in the polymer by varying both the length and the diameter of the tubes in the shell-and-tube heat exchanger. The residual monomer and oligomer remaining in the polymer after the Hagberg process is shown as ranging from 1000 to 110,000 PPM.
Hagberg U.S. Pat. No. 3,966,538 also issued Jun. 29, 1976 is a divisional case of the Hagberg patent above with claims directed to the apparatus rather than the process.
Newman U.S. Pat. No. 4,294,652 issued Oct. 13, 1981 discloses apparatus and process almost identical to the aforedescribed Hagberg devices. The only difference between the Newman device and the Hagberg devices is that the lower holding tank of Hagberg has been modified slightly by adding a pump-around pump and two baffles plus a vapor line out of the hold tank to achieve some slight additional devolatilizing in the lower holding tank. Although the Newman disclosure contains no specific examples of actual trial runs, Newman claims devolatilization of monomer content to less than about 100 to 500 PPM. No figures are stated for other volatiles contained in the finished polymer.
McCurdy et al U.S. Pat. No. 4,439,601 issued Mar. 27, 1984 discloses a process for devolatilization of mass processible polymers through the use of two or more flash devolatilizers in series and by utilizing a nonrefrigerated cooling process to condense the volatiles which are separated from the flash zones. The nonrefrigerated cooling process described uses a coolant such as water circulated through a heat exchanger to condense the volatiles in the multi-stage flash devolatilization. Each flash devolatilizer is operated at a lower pressure than the preceding one. No figures are given in the McCurdy disclosure for the levels of volatiles remaining in the finished polymer product.
Sosa et al U.S. Pat. No. 4,777,210 issued Oct. 11, 1988, discloses methods and apparatus for producing high-impact polystyrene having discrete particles of rubber in a styrene matrix. The process utilizes a preinversion reactor to closely control viscosity of the solutions and to produce desirable high-impact polystyrene products.
Sosa et al U.S. Pat. No. 4,857,587 issued Aug. 15, 1989, discloses methods and apparatus for producing high-impact polystyrene by removing inhibiting impurities from the recycle stream of the styrene polymerization zone.
Sosa et al U.S. Pat. No. 5,200,476 issued Apr. 6, 1993, discloses a system for reducing volatiles in a polymerization line, said system utilizing partial condensers, total condensers, devolatilizers, and filter beds arranged in series. The disclosures and specifications of the three aforementioned Sosa et al patents are herein incorporated by reference in their entirety into the present application.